Metal-air flow batteries using oxygen enriched electrolyte

ABSTRACT

A metal air flow battery includes an electrochemical reaction unit and an oxygen exchange unit. The electrochemical reaction unit includes an anode electrode, a cathode electrode, and an ionic conductive membrane between the anode and the cathode, an anode electrolyte, and a cathode electrolyte. The oxygen exchange unit contacts the cathode electrolyte with oxygen separate from the electrochemical reaction unit. At least one pump is provided for pumping cathode electrolyte between the electrochemical reaction unit and the oxygen exchange unit. A method for producing an electrical current is also disclosed.

CROSS-REFERENCE TO RELATED APPLICATION

This Application claims priority to U.S. Provisional Patent ApplicationNo. 61/641,676, filed May 2, 2012 and U.S. Provisional PatentApplication No. 61/766,455, filed Feb. 19, 2013, the entirety of whichare incorporated herein by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

This invention was made with government support under contract No.CERDEC/GTS-S-11-396 awarded by the U.S. Army and US Department of EnergyARPA-E program to response proposal solicitation #: DE-FOA-0000670. TheGovernment has certain rights in this invention.

FIELD OF THE INVENTION

The present invention relates to lithium ion batteries, and moreparticularly to metal air flow lithium ion batteries.

BACKGROUND OF THE INVENTION

Grid-connected renewable energy systems have experienced a rapid growthin the U.S. recently. Both wind and photovoltaic energy production havealmost doubled in the last several years requiring new energy storagesystems. As an example, the annual growth rates in the U.S. were 25% and74% for wind and photovoltaic energy, respectively, in 2011 over 2010.Due to the variable and stochastic nature of renewable sources thisenergy is difficult to manage, especially at high levels of penetration.The current lead-acid and flow batteries that are being used ingrid-connected renewable systems are not cost effective and reliableenough to be integrated in large grids. New storage solutions wouldultimately need to be scaled to tens of gigawatts of power with tens ofgigawatt-hours of energy distributed across the grid, to address theminutes-to-hours power firming and smoothing needed for renewable energygeneration nationwide.

Recently, Li-air batteries have been attracted much attention because ofthe possibility of extremely high energy density. The theoretical energydensity of the batteries can be over 3,000 Wh/kg which is more than 10times greater than that of Li-ion batteries. Although traditional Li-airbatteries have an extremely large theoretical energy density, theysuffer from several drawbacks: (1) the Li₂O₂/Li₂O discharge productdeposits on the air side of the electrode reducing the pore size andlimiting the access of O₂ into the cathode. The discharge productsdeposit mostly near the air side of the electrode because the O₂concentration is higher on this side. This inhomogenous deposition ofreaction products severely limits the usage of cathode volume, whichlimits the maximum capacity and energy density of the battery; (2) thecyclability and energy efficiency of Li-air batteries are poor due tothe lack of effective catalysts to convert solid Li₂O₂/Li₂O dischargeproducts into Li ions; and (3) the current and power densities of Li-airbatteries are much lower compared to conventional batteries due to theextremely low oxygen diffusion coefficient in liquid solution.

There are some efforts to improve the cyclability of Li-air batterieswith most research focusing on the development of catalysts which caneffectively accelerate the oxygen reduction process and reduce rechargeovervoltage. The poor reversibility of Li-air batteries is due to theformation of solid oxide discharge products which are difficult toreduce and decompose into Li-ions and oxygen within the electrolyte'sstable potential. Improved catalysts could reduce the reductionpotential but could not effectively reduce all solid oxide productsdeposited in a highly porous electrode. The most significant challengeto rechargeability of Li-air batteries is the formation of soliddischarge products.

SUMMARY OF THE INVENTION

A metal air flow battery comprises an electrochemical reaction unit. Theelectrochemical reaction unit comprises an anode electrode, a cathodeelectrode, an ionic conductive membrane between the anode and thecathode; an anode electrolyte; and a cathode electrolyte. An oxygenexchange unit is provided for contacting the cathode electrolyte withoxygen separate from the electrochemical reaction unit. At least onepump can be provided for pumping cathode electrolyte between theelectrochemical reaction unit and the oxygen exchange unit. The metalair flow battery can further comprise an electrolyte storage unit forreceiving cathode electrolyte from the electrochemical reaction unit andreturning cathode electrolyte to the electrochemical reaction unit.

The cathode electrode comprises a porous carbon. The porous carbon canbe at least one selected from the group consisting of carbon black,activated carbon, carbon nanotubes, carbon nanofibers, carbon fibers,and mixtures thereof.

The anode can be lithium metal. The anode can comprise at least oneselected from the group consisting of silicon, germanium, titanium,graphite carbon, and hard carbon.

The cathode electrolyte can be aqueous. The cathode electrolyte cancomprise at least one selected from the group consisting of LiOH,CH₃COOLi, LiClO₃, LiClO₄, HCOOLi, LiNO₃, C₆H₄(OH)COOLi, Li₂SO₄, LiBr,LiCl, LiSCN, and mixtures thereof.

The anode electrolyte can comprise a solvent selected from the groupconsisting of ethylene carbonate, propylene carbonate, dimethylcarbonate, diethyl carbonate, tetrahydrofuran, dimethoxyethane, andmixtures thereof. The anode electrolyte can comprises a salt selectedfrom the group consisting of lithium perchlorate, lithiumhexafluoroarsenate, lithium tetrafluoroborate, and mixtures thereof.

An ionic conductive membrane has good conductivity for Li ions and goodchemical stability in both non-aqueous and aqueous solutions. The ionicconductive membrane is also able to isolate the two electrolytes. Theionic conductive membrane can be a Celgard 2400 membrane.

The oxygen exchange unit can comprises an electrolyte storage unit. Theoxygen exchange unit can comprise a discharge manifold for dischargingoxygen into cathode electrolyte. The oxygen exchange unit can comprisesa plurality of stacked trays having apertures for the upward flow ofoxygen and the downward flow of cathode electrolyte. The oxygen exchangeunit can comprise an elongated conduit, the conduit comprising portionsthat are permeable to oxygen and impermeable to the cathode electrolyte.

The electrolyte entering the electrochemical reaction unit is caused toflow into one part of the porous cathode, flow through the porouscathode, and flow out of another side of the porous cathode.

A method for producing an electric current comprises the step ofproviding an electrochemical reaction unit comprising an anodeelectrode, a cathode electrode, an ionic conductive membrane between theanode and the cathode, an anode electrolyte, and a cathode electrolyte.An oxygen exchange unit contacts the cathode electrolyte with oxygenseparate from the electrochemical reaction unit. Cathode electrolyte ispumped between the electrochemical reaction unit and the oxygen exchangeunit and contacting the electrolyte with oxygen while the battery isbeing discharged. The cathode electrolyte can be caused to flow into onepart of the porous cathode electrode, flow through at least part of thecathode electrode to deliver O₂ to the cathode, and flow out of anotherpart of the cathode electrode prior to returning to the oxygen exchangeunit.

BRIEF DESCRIPTION OF THE DRAWINGS

There are shown in the drawings embodiments that are presently preferredit being understood that the invention is not limited to thearrangements and instrumentalities shown, wherein:

FIG. 1 is a schematic diagram showing the operational principle andconfiguration of a metal-air flow battery according to the invention.

FIG. 2 is a schematic diagram of an oxygen exchange unit.

FIG. 3 is a schematic diagram of an alternative embodiment with anelectrochemical reaction unit, an oxygen exchange unit, and anelectrolyte storage unit.

FIG. 4 is a plot of cell voltage as a function of the thickness of theair-electrode and current density.

FIG. 5 is a plot of electrical impedance spectroscopy (EIS) measuredfrom (a) 1 M LiOH solution in both electrolyte cells, (b) 1M LiPF₆ in PCin both electrolyte cells, and (c) 1 M LiOH solution and 1M LiPF₆ in PCin each electrolyte cell.

FIG. 6 is a plot of Li-air flow battery charge-discharge curves atvarious current densities.

FIG. 7 is a plot of power performance.

FIG. 8 is a plot of charge and discharge voltage difference at variouscurrent densities.

FIG. 9 is a plot of electrochemical impedance spectra after charge anddischarge at 5 mA cm⁻². The inset is the equivalent electric circuitused to fit EIS.

FIG. 10 is a schematic diagram of an alternative design of an oxygenexchange unit.

FIG. 11 is a schematic diagram of another alternative design of anoxygen exchange unit.

DETAILED DESCRIPTION OF THE INVENTION

A metal air flow battery includes at least an electrochemical reactionunit and an oxygen exchange unit. The electrochemical reaction unitincludes an anode electrode, a cathode electrode, an ionic conductivemembrane between the anode and the cathode, an anode electrolyte, and acathode electrolyte. The oxygen exchange unit contacts the cathodeelectrolyte with oxygen separate from the electrochemical reaction unit.The term “separate” as used herein means that the point of contactbetween O₂ diffusing into the cathode electrolyte is removed from thecathode electrode. The cathode electrolyte leaves the electrochemicalreaction unit to reach the oxygen exchange unit or the oxygenexchange/electrolyte storage combined unit (FIGS. 1 and 2), where it isenriched with O₂. Cathode electrolyte flows from the electrochemicalreaction unit to the oxygen exchange unit, and is enriched with O₂. Thecathode electrolyte is then returned from the oxygen exchange unit tothe electrochemical reaction unit after the cathode electrolyte. Thecathode electrolyte enriched with O₂ flows through the porous cathodeelectrode and supplies the O₂ to the electrochemical reaction. At leastone pump is provided for pumping cathode electrolyte between theelectrochemical reaction unit and the oxygen exchange unit.

The battery consists of at least two units (FIG. 1), the electrochemicalreaction unit 10, and the oxygen exchange unit 14. An electrolytestorage unit is also possible. The electrochemical reaction unit 10 hascathode electrode 18, anode electrode 22, membrane 26, cathodeelectrolyte 30, and anode electrolyte 34 which can be provided in asuitable battery case 38. The oxygen exchange/electrolyte storage unit14 can have cathode electrolyte inlet 42, and cathode electrolyte outlet46 in a container 50. An oxygen exchange device 54 releases oxygen intothe cathode electrolyte within the container 50. A pump 58 moves theelectrolyte between the electrochemical reaction unit 10 and the oxygenexchange unit 14. The electrochemical reaction unit during dischargeconverts chemical energy into electrical energy, and during chargingconverts electrical energy to chemical energy. The maximum output powerof the system is given by the maximum current density and the electrodesize of the electrochemical reaction unit; and the oxygen exchange unitregenerates (i.e. refreshes) the electrolyte to become electrochemicallyreactive. The electrolyte regeneration rate should preferably balancethe oxygen consumption rate in the electrochemical reaction unit. Theterm “balance” as used herein means that the molar rate of O₂ diffusinginto the cathode electrolyte equals the molar rate of consumption of O₂in the electrochemical reaction unit or is within 1%, 5%, 10%, 15%, 20%,or 25% of that rate.

The electrochemical reaction unit can be similar to conventional Li-airbatteries with dual electrolytes and can contain a Li metal (or other Lirich materials or Li intercalatable materials) as the anode material dueto its high specific capacity and low potential. A porous carbon orother suitable porous material or structure can be provided as the airelectrode. Many such materials and structures are known. A suitablesolid/polymer ionic conductive membrane is provided. An appropriateanode electrolyte and an appropriate cathode electrolyte are provided.The cathode electrode does not open directly to the atmosphere toreceive the oxygen, but instead electrolyte is circulated continuouslybetween the electrochemical reaction unit and the oxygen exchange unit(FIG. 1). For example, during discharge, the fresh electrolyte which issaturated with oxygen is pumped into the electrochemical reaction unit,while the used electrolyte will be removed from the electrochemicalreaction unit and sent to the oxygen exchange unit to be refreshed. Theoxygen exchange unit is designed to ensure the electrolyte will achievea satisfactory oxygen saturation level before entering into theelectrochemical unit.

The oxygen exchange unit can also be configured to store electrolyte.FIG. 2 shows one of possible designs. The aqueous electrolyte in theoxygen exchange unit 14 is bubbled with air from oxygen exchange device54 which can be any suitable oxygen exchange device in order to increasethe interfacial area between oxygen and electrolyte and make the oxygenconcentration in the electrolyte as high as possible. The housing orcontainer 50 of the oxygen exchange unit is sized to provide adequateelectrolyte storage to meet system requirements. The maximum energystorage capability of the system is ultimately determined by the amountof electrolyte in the electrolyte storage unit (or the size of thestorage container) and the solubility of the solvent(s) used for theelectrolyte.

The electrolyte storage unit and oxygen exchange unit in FIG. 1 can beseparated in order to make the system more energy efficient and easierto scale up. FIG. 3 shows a three-unit Li-air flow battery including theelectrochemical reaction unit 60, the oxygen exchange unit 64, and anelectrolyte storage unit 68. The electrochemical reaction unit 60 hascathode electrode 68, anode electrode 72, membrane 76, cathodeelectrolyte 80, and anode electrolyte 84 which can be provided in asuitable battery case 88. Cathode electrolyte enters through an inlet 92and leaves through an outlet 96. The oxygen exchange/electrolyte storageunit 64 can have cathode electrolyte inlet 102 and cathode electrolyteoutlet 104 in a container 110. An oxygen exchange device 114 releasesoxygen into the cathode electrolyte within the container 110. The oxygenexchange device 114 can be in the form of a flow canal which injects orotherwise permits the passage of oxygen into the cathode electrolyte.Flow through the oxygen exchange device 114 can be assisted by gravity120. The electrolyte storage unit 68 has a container 118 for storing thecathode electrolyte, an inlet 122 and an outlet 126. A pump 130 can movethe electrolyte between the electrochemical reaction unit 60 and theoxygen exchange unit 64, and the electrolyte storage unit 68.

It is important that during discharge, the fresh electrolyte which issaturated with oxygen is pumped into the electrochemical reaction unit,while the used electrolyte will be sent to the oxygen exchange unit tobe refreshed. The integrated exchange and storage system is designed toensure the electrolyte will achieve a satisfactory oxygen saturationlevel before entering into the electrochemical reaction unit. Theelectrolyte storage unit determines the maximum energy storage anddelivery capacity. The electrolyte storage unit can be made with severalsubunits. Subunits can be connected in series or parallel. The seriesconnection means that one inlet is connected to another outlet; theparallel connection means that inlets are connected together and outletsare connected together. The system can be pressurized.

The electrolyte storage unit is a container with at least one inlet andone outlet for circulation of the electrolyte as shown in FIG. 3. Theelectrolyte stored in the container is same as that in the cathodeelectrode of the electrochemical reaction unit. The electrolyte in theelectrochemical reaction unit will be cycled with the electrolytestorage unit during the charge and discharge.

During the discharge process, the oxygen rich electrolyte in the oxygenexchange unit will increase the oxygen concentration in the cathodeelectrolyte of the electrochemical reaction unit to provide enoughoxygen for the electrochemical reaction as:

4Li+O₂+2H₂O

4Li⁺+4H⁻  (1)

During the discharge process, the diluted electrolyte (Li concentration)in the electrolyte storage unit will reduce the Li ion concentration forpreventing the discharge product to reach the solubility limitation andsolid deposition in the cathode electrode of the electrochemicalreaction unit for preventing the discharge product to reach thesolubility limitation and solid deposition in the air electrode; duringthe charge (or re-charge) process, the electrolyte storage unit willprovide Li ions to the cathode electrode in the electrochemical reactionunit.

During the discharge process, the minimum flow rate of the cathodeelectrolyte through the cathode electrode in the electrochemicalreaction unit can be determined by the relationship of the currentproduced by the electrochemical reaction unit, the Li-ion concentrationand oxygen concentration in the electrolyte storage unit as:

$\begin{matrix}{{{{Flow}\mspace{14mu} {rate}\; 1}_{discharge}} = \frac{1}{F\left( {m_{sol} - m} \right)}} & (2) \\{{{{Flow}\mspace{14mu} {rate}\; 2}_{discharge}} = \frac{1}{2\; {Fm}_{O\; 2}}} & (3)\end{matrix}$

where l is the current, F is the Faraday constant and equals 96,485C/mol, m_(sol) is the maximum Li-ion molar concentration (solubility) ofthe electrolyte, m is the Li-ion molar concentration of the electrolyteleaving the oxygen exchange unit and/or the electrolyte storage unit,and m_(O2) is the oxygen molar concentration in the oxygen exchange unitand/or the electrolyte storage unit. The minimum cathode electrolyteflow rate will be determined by the greater value between Flow rate 1and Flow rate 2 in eqns. (2) and (3). The oxygen and waterconcentrations limit the reaction more than lithium-lithiumconcentration is not often a limiting factor. If there is not enoughwater the Li concentration will reach the solubility limit and start todeposit as a solid product. If m is close to m_(sol) then a much fasterwater/electrolyte flow will be necessary.

The minimum flow rate (usually liters/min) during the charging processcan be determined as:

$\begin{matrix}{{{{Flow}\mspace{14mu} {rate}}_{charge}} = \frac{1}{Fm}} & (4)\end{matrix}$

where F is the Faraday constant and m is the Li-ion molar concentration.A fast charge depends on the Li concentration.

The oxygen exchange unit is designed to allow the electrolyte from theelectrochemical reaction unit to be fully exposed to the air; therefore,the oxygen concentration in the electrolyte can be close to a saturationlevel, particularly during the discharge process. The electrolyte flowcanal as shown in FIG. 1 should be designed with a total length longenough that the electrolyte residence/flow time is longer than therequired time for the oxygen diffusion. The oxygen diffusion time can beestimated by:

$\begin{matrix}{t_{O\; 2} = \frac{l^{2}}{D_{O\; 2}}} & (5)\end{matrix}$

where l is the electrolyte depth in the electrode flow canal and theD_(O2) is the oxygen diffusion coefficient in the electrolyte.

The cathode electrode can be made with any suitable porous conductivecathode material, such as an electrically conductive porous carbon. Theporosity of the electrode will be optimized according to the electricalconductivity and the electrolyte flow resistance. The carbon used incathode can be carbon black, activated carbon, carbon nanotubes, carbonnanofibers, carbon fibers, and their mixture.

The cathode can be constructed so as to allow the cathode electrolyte toflow into the porous cathode and carry Li ions and O₂ to and from thecathode, and flow out of the porous cathode to return the cathodeelectrolyte to the oxygen exchange unit. High pressure drops should beavoided. The porous electrode should also be capable of retaining asuitable catalyst.

The electrolyte used in cathode electrode is preferably an aqueouselectrolyte. Water (H₂O) can be the solvent. Suitable electrolyte saltsfor the cathode include LiOH, CH₃COOLi, LiClO₃, LiClO₄, HCOOLi, LiNO₃,C₆H₄(OH)COOLi, Li₂SO₄, LiBr, LiCl, and LiSCN. Other possible solventsinclude methanol, acetonitrile, ethyl ether, acetone, ethanol, propanol,and isopropyl ether.

The anode electrode in the electrochemical reaction unit is made with Limetal, Li/other metal alloys, or Li/other metal mixtures. The othermetals can be silicon, germanium, titanium, graphite carbon, and hardcarbon. The solid anode material is surrounded by non-aqueouselectrolyte. The solid anode material can also be wrapped by a porouspaper.

The anode electrode can also be made without Li present, but withintercalatable materials such silicon, germanium, titanium, graphitecarbon, and hard carbon. In an anode electrode without Li present, theLi source (during charging) can come from the aqueous electrolyte incathode electrode. The Li ion will intercalate into the intercalationcomponents during the charging of the battery.

The anode electrolyte can be an organic electrolyte. The electrolyteused in the anode electrode can be similar to that used for conventionalLi-ion batteries. The electrolyte will be optimized as an electrolyte byforming it from an appropriate salt and an appropriate solvent mixture.The selection options include high dielectric constant carbonatesolvents such as ethylene carbonate (EC) and propylene carbonate (PC),which are able to dissolve sufficient amounts of lithium salt, lowviscosity carbonate solvents such as dimethyl carbonate (DMC) anddiethyl carbonate (DEC) for high ionic conductivity, and ether solventssuch as tetrahydrofuran (THF) dimethoxyethane (DME) for improved lithiummorphology in order to suppress dendritic lithium growth during thecycles. The selection of an appropriate salt for the anode electrolytecan be based on some conventional salts such as lithium perchlorate(LiClO₄), lithium hexafluoroarsenate (LiAsF₆), and lithiumtetrafluoroborate (LiBF₄), but not limit to these. Other anodeelectrolytes are possible.

The membrane between the anode and cathode electrodes must have a goodconductivity for Li ions and good chemical stability in both non-aqueousand aqueous solutions, as well as be able to isolate the twoelectrolytes. One such membrane is a Li-ion glass-ceramic (LIC-GC)membrane. Other membranes are possible, such as Li-ion conductivepolymers.

A separator such as the Celgard 2400 (Celgard LLC, Charlotte N.C.) canbe used. The separator is placed between the anode and the membrane.Other separators are possible, such as porous polyolefin based materialsincluding polyethylene, polypropylene, and their blends; graft polymersincluding micro-porous poly(methyl methacrylate) and siloxane graftedpolyethylene; poly(vinylidene fluoride) (PVDF) nanofiber webs;polytriphenylamine (PTPAn)-modified separator.

It is possible that impurities and dust from the air might diffuseinside and enter the electrolyte and clog the porous cathode in time.This problem can be addressed by using suitable air filters placed atthe air intakes. Other filters can be used at various points in thesystem to ensure a high purity of the water and/or other electrolyteliquids.

Catalysts can be used to increase the cathode potential during thedischarge and decrease it during the charging process, as discussed inthe last section. The round-trip energy efficiency can be improvedsignificantly by introducing bi-functional catalysts toincrease/decrease the cathode potential during discharge/recharge. Itwas found that nano-size α-MnO₂/carbon could lower the cathode potentialby more than 0.3 V, while reducing Li₂O₂ to 2Li++O₂ during the dischargein the organic electrolyte. The cathode electrode can comprises mixtureof porous carbons and catalysts. The catalysts can comprises at leastone selected from the group consisting of platinum, gold, silver, MnO₂,Ag₂Mn₈O₁₆, CeO₂, Y₂O₂SO₄, Gd₂O₂SO₄, La₂O₂SO₄, and mixtures thereof.

The new metal-air flow battery provides a comprehensive solution tosolve problems of traditional Li-air batteries such as low power(current) density and poor cyclability. The invention is not limited bycathode thickness because electrolyte flows through it carrying oxygen.Traditional cathodes are limited by O₂ diffusion. Diffusion is very slowin a liquid, and much faster when there is electrolyte flow. Priorefforts to flow electrolyte cause electrolyte to seep out of cathodeinto air. The invention avoids this problem by removing air andregenerating in the oxygen exchange unit. The invention allows forcathode thicknesses of at least 0.1 mm, 1 mm, 10 mm, 100 mm, or 500 mmto greater than 1 cm and thicknesses within these ranges. Prior artcathodes for Li-air batteries are practically limited to about 50-60microns to allow O₂ diffusion. The cyclability is significantly improvedby using a design with no solid product deposition at the cathode.

The initial aqueous electrolyte can be a diluted base (such as LiOH) oracid (CH₃COOH) solutions. For instance, in a base solution, the overallreaction described by eqn. (1) and the overall mass balance can beexpressed as:

Li+0.5O₂+.5H₂O+10.64H₂O

Li⁺+OH⁻+10.64H₂O  (6)

The discharge Li+OH⁻ product is formed at the surface of the cathodethrough a charge exchange process and is then dissolved in water. Themaximum concentration of Li+ and OH⁻ ions is determined by thesolubility of the LiOH in water which is 12.5 g of LiOH/100 g of water.The energy density of the Li-air flow battery can be estimatedconsidering that the solubility of LiOH in H₂O is 12.5 g of LiOH/100 gof H₂O at 25° C.; since 1 mol LiOH needs at least 10.64 mol of H₂O thespecific capacity is:

$\begin{matrix}\begin{matrix}{c_{p} = \frac{F}{M_{Li} + {0.5\; M_{H\; 2\; O}} + {10.64\; M_{H\; 2\; O}}}} \\{= \frac{96485\mspace{14mu} C\text{/}{mol}}{{6.94\mspace{14mu} g\text{/}{mol}} + {11.14 \times 18\mspace{14mu} g\text{/}{mol}}}} \\{= {465\mspace{14mu} C\text{/}g}}\end{matrix} & (7)\end{matrix}$

where M_(Li)=6.94 g/mol and M_(H2O)=18 g/mol. Since the operationalvoltage is V_(p)=3.69 V the estimated specific energy based on activematerials is E=c_(p)V_(p)=477 Wh/kg. In the case of Li-air flow batterywith an electrolyte made of diluted CH₃COOH solution, the theoreticalspecific energy is 483 Wh/kg. Including the mass of the carbon, currentcollector, package materials, and small pumps, the estimated practicalspecific energy of Li-air flow battery is 40%×E≈200 Wh/kg.

When lithium metal is used as the anode electrode, the theoreticalspecific energy as high as 483 Wh/kg can be achieved for the Li-air flowbattery; however, other materials can also be used as anode. Thegraphical carbon can be used as anode material and the electrochemicalreaction at the anode during charge and discharge will be:

C₆Li

C₆ +e ⁻+Li⁺  (8)

The theoretical specific capacity of Li-air flow batteries is:

$\begin{matrix}\begin{matrix}{c_{p} = \frac{F}{M_{C\; 6} + {0.5\; M_{H\; 2\; O}} + {10.64\; M_{H\; 2\; O}}}} \\{= \frac{96485\mspace{14mu} C\text{/}{mol}}{{72\mspace{14mu} g\text{/}{mol}} + {11.14 \times 18\mspace{14mu} g\text{/}{mol}}}} \\{= {354\mspace{14mu} C\text{/}g}}\end{matrix} & (9)\end{matrix}$

where, M_(C6)=72 g/mol is the molecular weight of C₆. If it is assumedthat the operational voltage is V_(p)=3.69 V, then the theoreticalspecific energy based on active materials is E=c_(p)V_(p)=363 Wh/kg.

Silicon is another high specific capacity material which can be used asanode. A specific capacity greater than 2000 mA/g has been achieved at areasonable good cyclability. The specific capacity of 2000 mA/gcorresponds to the fact that each silicon atom can intercalate (orreact) with 2 lithium ions; therefore, the theoretical specific capacityof Li-air flow batteries is:

$\begin{matrix}\begin{matrix}{c_{p} = \frac{F}{{\frac{1}{2}M_{Si}} + {0.5\; M_{H\; 2\; O}} + {10.64\; M_{H\; 2\; O}}}} \\{= \frac{96485\mspace{14mu} C\text{/}{mol}}{{\frac{1}{2} \times 28\mspace{14mu} g\text{/}{mol}} + {11.14 \times 18\mspace{14mu} g\text{/}{mol}}}} \\{= {450\mspace{14mu} C\text{/}g}}\end{matrix} & (10)\end{matrix}$

The current density of new metal-air batteries was also estimated usingfinite element simulations to solve the transport equations in theelectrochemical reaction unit. FIG. 4 shows the cell voltage asfunctions of current density (J) and the thickness of the air electrode.It was found that for an air electrode with a thickness of 1 cm, thecell voltages are 3.31, 3.22, and 2.64 V at current densities of 0.5,1.0, and 5.0 mA/cm², respectively, when the electrolyte is saturated inair at 1 atm. These current densities are 10-100 times larger than thoseachieved by traditional Li-air batteries. An advantage of this metal-airflow battery is that the cathode can be as thick as 1 cm and filled withelectrolyte saturated with oxygen. The amount of oxygen available intraditional Li-air batteries is mainly limited by the effective oxygendiffusion length λ=2F∈^(1.5)c_(O2)D_(O2)/J, where ∈ and c_(O2) are theporosity and oxygen concentration in the air electrode, and D_(O2) isthe oxygen diffusion coefficient in the electrolyte. The effectivethicknesses of the air electrode are only 13.6, 6.8, and 1.36 μm forJ=0.5, 1.0, and 5.0 mA/cm², respectively, for 1 atm. air.

An important factor which limits the current density of the metal-airbatteries is the voltage loss due to the ohmic resistance of themembrane. The membrane must not only have a good conductivity for Liions but also good chemical stability in both non-aqueous and aqueoussolutions, as well as be able to isolate the two electrolytes. Theresistance of the membrane of such as Li-ion conducting glass-ceramic(LIC-GC) and of the interface between LIC-GC and the electrolytes weremeasured using the electrochemical impedance spectral (EIS) method. TheLIC-GC from Ohara Inc. is a 50 μm thick membrane with a size of2.54×2.54 cm² and was placed and sealed between two electrolyte cells.Each electrolyte cell has an open window of 1×1 cm². A Pt electrode wasplaced in each electrolyte cell. Two different electrolytes were used −1M LiOH in H₂O and 1M LiPF₆ in propylene carbonate (PC), respectively.The EIS was measured between two Pt electrodes at a frequency range of 1Hz to 100 kHz using a Solartron 1250B frequency response analyzercontrolled by Zplot and Corrware software. FIG. 5 is the EIS measuredfrom (a) 1M LiOH solution in both electrolyte cells, (b) 1M LiPF6 in PCin both electrolyte cells, and (c) 1 M LiOH solution and 1M LiPf6 in PCin each electrolyte cell. FIG. 5 shows the EIS measured with differentelectrolytes in two electrolyte cells. The bulk resistance of the LIC-GCmembrane and interfacial resistance between the membrane and electrolytewere obtained by fitting EIS using an equivalent electric circuit asshown in FIG. 5 (inset). R_(b) represents the total bulk resistancesfrom LIC-GC membrane and electrolytes. The two R and C pairs representinterfacial resistances at the two membrane interfaces. The resistancecontributed by electrolytes was measured by removing the LIC-GC membranewhen the same electrolyte was filled in both electrolyte cells andsubtracted from the EIS as shown in FIG. 5. The resistance distributioncan be summarized as follows: The bulk resistance of the LIC-GC membranewas R_(b)˜50 Ω-cm² which is equivalent to a conductivity of 1×10⁻⁴ S/cm;the interfacial resistances were R_(int1)˜70 Ω-cm² with the aqueouselectrolyte (1 M LiOH solution) and R_(int2)˜80 Ω-cm² with thenon-aqueous electrolyte (1M LiPF₆ in PC), respectively; therefore, thetotal cell resistance at low frequencies was about 200 Ω-cm².

A battery according to the invention consists of a lithium-ionconducting glass-ceramic membrane sandwiched by a Li-metal anode inorganic electrolyte and a carbon nanofoam cathode through whichoxygen-saturated aqueous electrolyte flows. It features a flow celldesign in which aqueous electrolyte is bubbled with compressed air, andis continuously circulated between the cell and a storage reservoir tosupply sufficient oxygen for high power output. It shows high ratecapability (5 mA cm⁻²) and renders a power density of 7.64 mW cm⁻² at aconstant discharge current density of 4 mA cm⁻².

The maximum output power of the system is given by the maximum currentdensity and the electrode size of the electrochemical reaction unit; theelectrolyte storage unit determines the maximum energy storage anddelivery capacity; and the oxygen exchange unit regenerates theelectrolyte to become electrochemically reactive. The theoretical energydensities of these rechargeable Li-air flow batteries vary from 140 toover 1100 Wh/kg depending on the type of electrolytes in cathode. One ofthe advantages of Li-air flow batteries is that the energy and powercapabilities can be totally separated according to the loadrequirements.

Example

An experimental Li-air flow battery was prepared. The cathode electrodedoes not open directly to the atmosphere to receive the oxygen; insteadit circulates the electrolyte continuously between the electrochemicalreaction unit and electrolyte storage unit (FIG. 3). For example, duringdischarge, the fresh electrolyte which is saturated with oxygen ispumped into the electrochemical reaction unit, while the usedelectrolyte will be sent to the oxygen exchange unit to be refreshed. Inthe Li-air flow battery, the anode is a piece a Li-metal foil (AlfaAesar, 99.9%, 0.75 mm thickness). A piece of separator (Celgard 2400,Celgard LLC, Charlotte N.C.) was placed between a Li-metal foil and alithium-ion conducting glass-ceramic (LIC-GC) membrane (Ohara Inc., 0.15mm thickness). The cathode is a carbon nanofoam. The aqueous electrolyteat the cathode and electrolyte container was made with 0.85 M CH₃COOH(HOAc, Sigma Aldrich, ≧99.7%) and CH₃COOLi (LiOAc, Sigma Aldrich) indeionized water. The organic electrolyte at the anode was 1.2 M LiPF₆ inethylene carbonate (EC)/dimethyl carbonate (DMC) at a ratio of 1:1 byweight as received (Novolyte Technologies Inc.).

Li-metal foil was roll-pressed onto the copper mesh, which acts as theanode. The LIC-GC was sealed on to an aluminum laminated polymer,leaving a window area of 3.61 cm² of LIC-GC open for lithium ionmovement during discharge and charge processes. The aluminum laminatedpolymer that was used is a very flexible material accommodating for anyvolume changes in Li-metal anode. A glass sheet is used in conjunctionwith a stainless steel spring to put uniform and continuous pressure onthe layered battery structure.

A piece of carbon nanofoam was used as the cathode electrode and wasmade as follows: The resorcinol (>99%) and sodium carbonate weredissolved in DI water, with stirring for fifteen minutes. Thepolymerization was initiated by introducing formaldehyde solution (37wt. %) into the stirred solution to form the precursor solution. Theratio of resorcinol and formaldehyde is 1:2. In the precursor solution,a small amount of sodium carbonate catalyst was added. The precursorsolution was filled into a stack of carbon fiber papers (from Lydall,density 0.2 g/cm³, 90 um thick) which was placed between two glasses. Arubber O-ring was used to control the thickness of the carbon fiberpaper and also acted as a sealant. After the filling process, thematerials inside glass plate container were solidified by remained atroom temperature for 2 days, then 80° C. for 2 days. The samples weredried at 50° C. under ambient pressure after an exchange of the poreliquid for acetone, then pyrolyzed under a nitrogen atmosphere at 1000°C. in a tube furnace. The furnace was purged with nitrogen at roomtemperature for one hour, and then ramped to 1000° C. at 5° C./min. Thetemperature remained at 1000° C. for two hours before returning to roomtemperature.

The electrolyte storage unit was a stainless steel container about 1 Lin volume. Compressed air was bubbled into the aqueous electrolyte by agas bubbler (McMaster-Carr). Dissolved oxygen (DO) and pH in aqueouselectrolyte were measured to be 8.3 mg L⁻¹ and 4.5 by an Oakton HandheldMeter (PCD 650) and a Mettler-Toledo pH Meter, respectively. The aqueouselectrolyte was circulated by a VWR variable flow mini-pump (Model 3389)at the speed of 250 mL min⁻¹. Charge and discharge measurements werecarried out in air atmosphere at room temperature using an ArbinInstruments (Arbin-010 MITS pro 4.0-BT2000) controlled by a computer.The electrochemical impedance spectrum of the Li-air flow battery wasrecorded over a frequency sweep of 0.1-10⁶ Hz using a Gamry Instruments(Reference 3000). The resulting spectrum was analyzed by Gamry EchemAnalyst program.

FIG. 6 displays the charge-discharge curves at various currentdensities. The discharge and charge voltages keep at 3.2 V and 3.9 V,respectively, at a current density of 1 mA cm⁻². Even at the currentdensity of 5 mA cm⁻², the discharge and charge voltages still keep at1.5 V and 5.2 V, respectively. With the growth of applied currentdensity, the discharge voltage linearly decreases, while its powerdensity sharply increases as shown in FIG. 7. At the current density of4 mA cm⁻², the Li-air flow battery reaches its maximal power density of7.64 mW cm⁻². As the applied current density grows, the linear increaseof charge and discharge voltage difference is clearly observed in FIG.8. The resistance of the Li-air flow battery calculated from the linearfit equation is about 374 Ωcm². A current density of 5 mA is not commonand is only limited by membrane resistance. The invention will permithigher current densities—the current density could go higher given thatohmic resistance is limiting factor.

The electrochemical impedance spectra (EIS) of a Li-air flow battery isrecorded after charge and discharge at 5 mA cm⁻² at a frequency range of0.1-10⁶ Hz in FIG. 9. An equivalent electric circuit is used to simulatethe EIS, as shown in the inset of FIG. 9. The total resistance of cellmembrane is 200 of the 250 so this resistance is clearly the limitingfactor to current density performance. In the circuit, the highfrequency intercept of the semicircle on the real axis is reflected byan ohmic resistance (R_(s)), which is predominantly the bulk resistanceof the LIC-GC. The large semicircle in the high frequency rangerepresents the interfacial resistance (R_(int)) of the LIC-GC with bothorganic electrolyte and aqueous electrolyte. The small semicircle in themiddle frequency range corresponds to (i) the resistance of apassivation film (R_(f)) on the lithium electrode surface, and (ii) thecharge-transfer resistance (R_(ct)). Three constant phase elements(Z_(Q)) are in parallel with the resistance. An inclined line in the lowfrequency range is related to a finite length Warburg element (Z_(w))arising from a diffusion-controlled process. The O₂ diffusion resistanceis negligible due to the high speed circulation of O₂-saturated aqueouselectrolyte. The fitting parameters are listed in Table 1. It can beseen that the total resistance is comparable to the value calculatedfrom the linear fit equation in FIG. 8. From the fitted results, theohmic resistance (R_(s)) is approximately 90 Ωcm², and the interfacialresistance (R_(int)) is around 115 Ωcm². These values are consistentwith previous measurements of the ionic resistance and the interfaceresistances from the LIC-GC. These two resistances which take up a majorpart in the total resistance are mainly attributed to the LIC-GC.Accordingly, higher power performance could be achieved by reductions inthe resistance of the Li-ion conducting glass ceramic.

In some Li-air flow batteries, Li-metal foil is used as the anodeelectrode. The safety of the Li metal is always an importantconsideration. Table 2 shows theoretical energy densities of Li-air flowbatteries if different anode materials such as Li metal, silicon, andgraphite carbon are used. The theoretical specific energy was calculatedbased on EC reaction as:

4Li+O₂+4CH₃COOH

4CH₃COOLi+2H₂O  (1)

Since the solubility of the discharge product CH₃COOLi is 45 g CH₃COOLiper 100 g H₂O; therefore the maximum specific capacity can be calculatedby:

$\begin{matrix}{c_{p} = \frac{F}{M_{anode} + M_{C_{2}H_{4}O_{2}} + {7.65\; M_{H_{2}O}}}} & (2)\end{matrix}$

where, F is the Faraday constant. M_(anode) is the molecular weight ofthe anode material are 6.94 g/mol, 13.3 g/mol, 79 g/mol for Li metal,silicon, and graphite carbon, respectively. M_(C2H4O2) and M_(H2O) arethe molecular weight of CH₃COOH and water, respectively. The theoreticalspecific energy was calculated as:

∈=c _(p) ×V  (3)

A cell voltage of 3.6 V was used.

TABLE 1 Fitting parameters of EIS curve fitting R_(s)(Ω R_(int)(Ω Z_(Q1)R_(f)(Ω Z_(Q2) R_(ct)(Ω Z_(Q3) Z_(w) cm²) cm²) Q₀(Ω⁻¹ s^(α)) α cm²)Q₀(Ω⁻¹ s^(α)) α cm²) Q₀(Ω⁻¹ s^(α)) α R_(w)(Ω cm²) T_(w)(S) 89.9 115.32.7 × 10⁻⁷ 0.96 34.1 1.1 × 10⁻⁶ 0.79 19.2 1.1 × 10⁻⁴ 0.81 144.4 27.1

TABLE 2 The theoretical specific energy of Li-air flow batterieswithdifferent anode electrode materials Theoretical Cell Anode SpecificCapacity Specific Energy Anode Material (mAh/g) (Wh/kg) Li 3862 477SiLi_(4.4)  371 (C) 463 C₆Li 4200 (Si) 354

The oxygen exchange unit can be of any suitable design. FIG. 10 showsanother possible design for the oxygen exchange unit 136 in which aseries of trays 140 can contain cathode electrolyte 144 and allows theelectrolyte to fall as drops 148 such that the drops are contacted withoxygen. An inlet 150 and outlet 158 are provided. FIG. 11 shows analternative oxygen exchange unit 170. The cathode electrolyte from theelectrochemical reaction unit can be fed into an oxygen exchange unitmade of a tube 174 with various shapes such as the coil shown in FIG.11. The cathode electrolyte will flow into the tube 174 from a higherinlet 178 to a lower outlet 182, or can be pumped under pressure. Thetube 174 can be made with some open holes at the upper surface to allowair diffusion into the tube and the cathode electrolyte flowing therein.The tube 174 can also be made with materials which have the propertythat oxygen can permeate through the tube wall, however, the liquidelectrolyte cannot permeate out from through the tube wall. Otherdesigns are possible.

The metal-air flow battery of the invention will have a significantimpact on the grid-scale energy storage because: (1) the cost ofmetal-air flow batteries will be significantly lower compared to otherbatteries; (2) the energy density of the proposed metal-air flowbatteries is above 200 Wh/kg, which is much higher than that of existingflow, liquid-metal, lead-acid, or advanced Li-ion batteries; (3) themetal-air flow batteries of the invention are different fromconventional batteries in which the maximum energy storage and powerdeliverable are proportional to the weight of the battery, and theenergy and power capabilities can be totally separated according to theload requirements. In metal-air flow batteries, the total energy storageis determined by the volume of the electrolyte storage unit and themaximum power capability is determined by the size and design of theelectrochemical reactor unit; (4) the manufacture shipment andinstallation weight of metal-air flow batteries is low, because only thereactor, which accounts for <20% of the total weight of the battery,needs to be pre-installed. The major weight of the battery is water,which can be introduced in the battery on the site, after theinstallation.

This invention can be embodied in other forms without departing from thespirit or essential attributes thereof, and reference should thereforebe had to the following claims as indicating the scope of the invention.

We claim:
 1. A metal air flow battery, comprising: an electrochemical reaction unit, comprising: an anode electrode, a cathode electrode, and an ionic conductive membrane between the anode and the cathode; an anode electrolyte; and, a cathode electrolyte; an oxygen exchange unit for contacting the cathode electrolyte with oxygen separate from the electrochemical reaction unit; and, at least one pump for pumping cathode electrolyte between the electrochemical reaction unit and the oxygen exchange unit.
 2. The metal air flow battery of claim 1, further comprising an electrolyte storage unit for receiving cathode electrolyte from the electrochemical reaction unit and returning cathode electrolyte to the electrochemical reaction unit.
 3. The metal air flow battery of claim 1, wherein the cathode electrode comprises a porous carbon.
 4. The metal air flow battery of claim 3, wherein the porous carbon comprises at least one selected from the group consisting of carbon black, activated carbon, carbon nanotubes, carbon nanofibers, carbon fibers, and mixtures thereof.
 5. The metal air flow battery of claim 4, wherein the cathode electrode comprises mixture of porous carbon and catalysts.
 6. The metal air flow battery of claim 5, wherein the catalyst comprises at least one selected from the group consisting of platinum, gold, silver, MnO₂, Ag₂Mn₈O₁₆, CeO₂, Y₂O₂SO₄, Gd₂O₂SO₄, La₂O₂SO₄, and mixtures thereof.
 7. The metal air flow battery of claim 1 wherein the anode is lithium metal.
 8. The metal air flow battery of claim 1, wherein the anode comprises at least one selected from the group consisting of silicon, germanium, titanium, graphite carbon, and hard carbon.
 9. The metal air flow battery of claim 1, wherein the cathode electrolyte is aqueous.
 10. The metal air flow battery of claim 1, wherein the electrolyte comprises at least one selected from the group consisting of LiOH, CH₃COOLi, LiClO₃, LiClO₄, HCOOLi, LiNO₃, O₆H₄(OH)COOLi, Li₂SO₄, LiBr, LiCl, LiSCN, and mixtures thereof.
 11. The metal air flow battery of claim 1, wherein the anode electrolyte comprises a solvent selected from the group consisting of ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, tetrahydrofuran, dimethoxyethane, and mixtures thereof.
 12. The metal air flow battery of claim 1, wherein the anode electrolyte comprises a salt selected from the group consisting of lithium perchlorate, lithium hexafluoroarsenate, lithium tetrafluoroborate, and mixtures thereof.
 13. The metal air flow battery of claim 1, wherein the ionic conductive membrane comprises Celgard
 2400. 14. The metal air flow battery of claim 1, wherein the oxygen exchange unit comprises an electrolyte storage unit.
 15. The metal air flow battery of claim 1, wherein the oxygen exchange unit comprises a discharge manifold for discharging oxygen into cathode electrolyte.
 16. The metal air flow battery of claim 1, wherein the oxygen exchange unit comprises a plurality of stacked trays having apertures for the upward flow of oxygen and the downward flow of cathode electrolyte.
 17. The metal air flow battery of claim 1, wherein the oxygen exchange unit comprises an elongated conduit, the conduit comprising portions that are permeable to oxygen and impermeable to the cathode electrolyte.
 18. The metal air flow battery of claim 1, wherein electrolyte entering the electrochemical reaction unit is caused to flow into one part of the porous cathode, flow through the porous cathode, and flow out of another side of the porous cathode.
 19. A method for producing an electric current, comprising the steps of: providing an electrochemical reaction unit, comprising an anode electrode, a cathode electrode, an ionic conductive membrane between the anode and the cathode, an anode electrolyte, and a cathode electrolyte; providing an oxygen exchange unit for contacting the cathode electrolyte with oxygen separate from the electrochemical reaction unit; and, pumping cathode electrolyte between the electrochemical reaction unit and the oxygen exchange unit and contacting the electrolyte with oxygen while the battery is being discharged.
 20. The method of claim 19, wherein the cathode electrolyte is caused to flow into one part of the porous cathode electrode, flow through at least part of the cathode electrode, and flow out of another part of the cathode electrode prior to returning to the oxygen exchange unit. 